Add 404 image

[Wilovy09]

Please read CONTRIBUTING.md before contributing or starting work

We need to implement the 404 image in the 404.astro view so that when the user arrives there, they don't just see a button with a “<-” but at least see an image.

The image is located in “/src/assets/”. It is the image we propose, but we are open to other options.

There is room for creative freedom here, as long as the main colors of the platform are retained.